Synthetic lubricating oils

ABSTRACT

A synthetic lubricating oil comprising as a base oil at least one kind of a carbonic acid ester selected from the group consisting of: 
     [I] a carbonic acid ester represented by the general formula (1) ##STR1##  wherein R 1  and R 2  are each an alkyl or dihydric alcohol residue, R 3  is an alkylene, and a is 0-30, 
     [II] a carbonic acid ester represented by the general formula (2) ##STR2##  wherein X 1  is --H or ##STR3##  R 4  -R 6  are each an alkyl, cycloalkyl or neopentyl type polyol residue, R 7  is an alkyl, b is 1 or 2, c is 0 or 1 (b+c=2), d is 0-30, and e is 1-3, and 
     [III] a carbonic acid ester of which alcohol moieties are (A) a monohydric alcohol, (B) a dihydric alcohol, and (C) a neopentyl type polyol or dimer or trimer thereof.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a synthetic lubricating oil and, morespecifically, it relates to a synthetic lubricating oil which comprisesa specific carbonic acid ester as a major component (a base oil) and isespecially useful as a lubricating oil for refrigerators and the like(the oil being hereinafter referred to as "a refrigerator oil").

2. Prior Art

Generally, naphthenic mineral oils, paraffinic mineral oils,alkylbenzenes, polyglycolic oils and mixtures thereof, which have each akinematic viscosity of 10-200 cSt at 40° C., as well as said oilsincorporated with suitable additives, have been used as refrigeratoroils.

On the other hand, chlorofluorocarbon type refrigerants (CFCS), such asCFC-11, CFC-12, CFC-115 and HCFC-22, have been used as such forrefrigerators.

Of these CFCS, those such as CFC-11, CFC-12 and CFC-115, which areobtained by substituting all the hydrogen atoms of their hydrocarbons byhalogen atoms including chlorine atoms, may lead to the destruction ofthe ozone layer, and, therefore, the use of the CFCS has beenrestricted. Accordingly, hydrogen-containing halogenocarbons,particularly chlorine-free type halogenocarbons such as HFC-134a andHFC-152a, have been being used as substitutes for CFCS. HFC-134a isespecially hopeful as a substitute refrigerant since it is similar inthermodynamic properties to CFC-12 which has heretofore been used inmany kinds of refrigerators for home cold-storage chests,air-conditioners, car air-conditioners and the like.

Refrigerator oils require various properties, among which theirmiscibility with refrigerants is extremely important in regard tolubricity and system efficiency in refrigerators. However, conventionalrefrigerator oils comprising, as the base oils, naphthenic oils,paraffinic oils, alkylbenzenes and the like, are hardly miscible withchlorine-free type halogenocarbons such as HFC-134a. Therefore, if saidconventional refrigerator oils are used in combination with HFC-134a,the resulting mixture will separate into two layers at normaltemperature thereby to degrade the oil-returnability which is the mostimportant within the refrigeration system and cause various troublessuch as a decrease in refrigeration efficiency, the deterioration oflubricity and the consequent seizure of the compressor within the systemwhereby the refrigerator oils are made unsuitable for use as such.

In attempts to solve such problems, the present inventors developedpolyglycolic refrigerator oils which have excellent miscibility withHFC-134a as compared with conventional known refrigerator oils, filed anapplication for a patent for the thus developed polyglycolicrefrigerator oils and have already obtained a patent (U.S. Pat. No.4,948,525) therefor. Further, U.S. Pat. No. 4,755,316 disclosespolyglycolic refrigerator oils which are compatible with HFC-134a.

On the other hand, refrigerator oils used in compressors of homerefrigerators and the like are required to have a high electricalinsulating property. Among the known refrigerator oils, alkylbenzenesand the mineral oils have the highest insulating property, but they arehardly miscible with chlorine-free type halogenocarbons such as HFC-134aas mentioned above. Further, the polyglycolic oils having excellentmiscibility with HFC-134a raise problems as to their inferior electricalinsulating property.

The present inventors found that esters having specific structures haveexcellent miscibility with chlorine-free type halogenocarbons such asHFC-134a and a high electrical insulating property, and filedapplications for patents for the esters (Japanese Patent ApplicationsNos. Hei. 1-341244, Hei. 1-341245, Hei. 2-105772 and Hei. 2-121133. Hei.1 and Hei. 2 are the same as 1989 and 1990 A.D., respectively.).

The known esters have various excellent properties as mentioned above,but they are hydrolyzable and therefore they are not necessarilysuitable for use in apparatuses, such as cold-storage chests and carair-conditioners, which must be operated for a long period of time withhigh reliability. Accordingly, it has been desired to develop arefrigerator oil which has high miscibility with hydrogen-containinghalogenocarbons such as HFC-134a, and also has a high electricalinsulating property and high hydrolysis stability (or high stability tohydrolysis).

Further, conventional ester type synthetic oils have heretofore beenknown as synthetic oils having excellent heat resistance, but they arehydrolyzable as mentioned above, and therefore, it has been desired todevelop an ester type synthetic oil having excellent hydrolyticstability.

SUMMARY OF THE INVENTION

The present inventors made various intensive studies in attempts todevelop lubricating oils which can meet the aforesaid requirements and,as the result of their studies, they found that carbonic acid estershaving specific structures have excellent miscibility with not onlychlorine type halogenocarbons such as CFC-12, HCFC-22 and HCFC-142b, butalso chlorine-free type halogenocarbons such as HFC-134a and HFC-152a,and a high electrical insulating property as well as excellent lubricityand excellent hydrolytic stability. This invention is based on thisfinding.

The object of this invention is to provide synthetic lubricating oilswhich comprise as a major component (or a base oil) at least one kind ofcarbonic acid ester having a specific structure, and have excellentmiscibility with hydrogen-containing halogenocarbons such as HFC-134a,and a high electrical insulating property as well as excellenthydrolysis stability.

The synthetic lubricating oil of the present invention is characterizedin that it comprises as a base oil at least one kind of a carbonic acidester selected from the group consisting of:

[I] a carbonic acid ester represented by the general formula (1)##STR4##

[II] a carbonic acid ester represented by the general formula (2)##STR5## and

[III] a carbonic acid ester of which alcohol moieties are

(A) a monohydric alcohol,

(B) a dihydric alcohol, and

(C) a neopentyl type polyol having 5-7 carbon atoms and 3-4 hydroxylgroups, or a dimer or trimer of the polyol.

First, the carbonic esters [I] will be explained in detail as indicatedbelow.

In the formula (1), R¹ and R² may be identical with, or different from,each other and are each a group selected from the group consisting of aalkyl groups having 1-15 carbon atoms, preferably 2-9 carbon atoms, anddihydric alcohol residues having 2-12 carbon atoms, preferably 2-9carbon atoms, R³ is an alkylene group having 2-12 carbon atoms,preferably 2-9 carbon atoms, and a is an integer of 0-30, preferably1-30.

The alkyl groups having 1-15 carbon atoms represented by each of R¹ andR² are exemplified by methyl group, ethyl group, n-propyl group, n-butylgroup, n-pentyl group, n-hexyl group, n-heptyl group, n-octyl group,n-nonyl group, n-decyl group, n-undecyl group, n-dodecyl group,n-tridecyl group, n-tetradecyl group, n-pentadecyl group, iso-propylgroup, iso-butyl group, tert.-butyl group, iso-pentyl group, iso-hexylgroup, iso-heptyl group, iso-octyl group, iso-nonyl group, iso-decylgroup, iso-undecyl group, iso-dodecyl group, iso-tridecyl group,iso-tetradecyl group and iso-pentadecyl group. Further, the dihydricalcohol residues having 2-12 carbon atoms are exemplified by theresidues of ethylene glycol, 1,3-propanediol, propylene glycol,1,4-butanediol, 1,2-butanediol, 2-methyl-1,3-propanediol,1,5-pentanediol, neopentyl glycol, 1,6-hexanediol,2-ethyl-2-methyl-1,3-propanediol, 1,7-heptanediol,2-methyl-2-propyl-1,3-propanediol, 2,2-diethyl-1,3-propanediol,1,8-octanediol, 1,9-nonanediol, 1,10-decanediol, 1,11-undecanediol and1,12-dodecanediol. The alkylene groups having 2-12 carbon atomsrepresented by R³ are exemplified by straight-or branched-chain alkylenegroups, such as ethylene group, trimethylene group, propylene group,tetramethylene group, butylene group, 2-methyltrimethylene group,pentamethylene group, 2,2-dimethyltrimethylene group, hexamethylenegroup, 2-ethyl-2-methyltrimethylene group, heptamethylene group,2-methyl-2-propyltrimethylene group, 2,2-diethyltrimethylene group,octamethylene group, nonamethylene group, decamethylene group,undecamethylene group and dodecamethylene group.

The method for producing the carbonic acid ester [I] used in the presentinvention is not particularly limited. For example, a carbonic acidester [I] can be produced by reacting in the presence of an alkali (suchas metallic sodium, sodium hydroxide or sodium methoxide) and at atemperature of 80°-200° C. an alcohol having 1-15 carbon atoms such asmethanol, ethanol, propanol, butanol, pentanol, hexanol, heptanol,octanol, nonanol, decanol, undecanol, dodecanol, tridecanol,tetradecanol or pentadecanol, with a carbonic acid diester or phosgene,and a dihydric alcohol such as ethylene glycol, 1,3-propanediol,propylene glycol, 1,4-butanediol, 1,2-butanediol,2-methyl-1,3-propanediol, 1,5-pentanediol, neopentyl glycol,1,6-hexanediol, 2-ethyl-2-methyl-1,3-propanediol, 1,7-heptanediol,2-methyl-2-propyl-1,3-propanediol, 2,2-diethyl-1,3-propanediol,1,8-octanediol, 1,9-nonanediol, 1,10-decanediol, 1,11-undecanediol,1,12-dodecanediol or trimethylene glycol. Of course, these alcohols,carbonic acid diesters and dihydric alcohols may be used jointly as amixture of at least two kinds thereof.

Secondly, the carbonic esters [II] will be hereunder explained indetail.

In the formula (2), X¹ is a hydrogen atom or a group represented by thegeneral formula ##STR6## R⁴, R⁵ and R⁶ may be identical with, ordifferent from, one another and are each a group selected from the groupconsisting of alkyl groups having 1-12 carbon atoms, preferably 3-7carbon atoms, cycloalkyl groups having 5-12 carbon atoms, preferably 5-8carbon atoms, and neopentyl type polyol residues having 5-30 carbonatoms, preferably 5-21, and 3-8 hydroxyl groups; R⁷ is an alkyl grouphaving 1-6 carbon atoms, preferably 1-4 carbon atoms; b is an integer of1 or 2, and c is an integer of 0 or 1 with the proviso that the sum(b+c)=2; d is an integer of 0-30, preferably 1-30, (when d is an integerof 0, at least one of R⁴ and R⁵ is the residue of a neopentyl typepolyol); and e is an integer of 1-3.

The alkyl groups having 1-12 carbon atoms represented by R⁴ -R⁶ areexemplified by methyl group, ethyl group, n-propyl group, iso-propylgroup, n-butyl group, iso-butyl group, sec.-butyl group, tert.-butylgroup, n-pentyl group, iso-pentyl group, neo-pentyl group, n-hexylgroup, iso-hexyl group, n-heptyl group, iso-heptyl group, n-octyl group,iso-octyl group, n-nonyl group, iso-nonyl group, n-decyl group,iso-decyl group, n-undecyl group, iso-undecyl group, n-dodecyl group andiso-dodecyl group. Further, the cycloalkyl groups having 5-12 carbonatoms may further include alkylcycloalkyl groups and are exemplified bycyclopentyl group, cyclohexyl group, cycloheptyl group, cyclooctylgroup, cyclononyl group, cyclodecyl group, cycloundecyl group,cyclododecyl group, methylcyclohexyl group, ethylcyclohexyl group,propylcyclohexyl group, butylcyclohexyl group and pentylcyclohexylgroup. The neopentyl type polyol residues having 5-30 carbon atoms and3-8 hydroxyl groups are exemplified by the residues oftrimethylolethane, trimethylolpropane, trimethylolbutane,pentaerythritol and a dimer or trimer thereof. The alkyl groups having1-6 carbon atoms represented by R⁷ are exemplified by methyl group,ethyl group, propyl group, n-propyl group, iso-propyl group, n-butylgroup, iso-butyl group, sec.-butyl group, tert.-butyl group, n-pentylgroup, iso-pentyl group, neo-pentyl group, n-hexyl group and iso-hexylgroup.

The method for producing the carbonic acid ester [II] used in thepresent invention is not particularly limited. For example, the carbonicacid ester [II] can be produced by reacting a chloroformate having 1-12carbon atoms with pentaerythritol, trimethylolethane,trimethylolpropane, trimethylolbutane, a dimer or trimer thereof, or amixture thereof, in the presence of an alkali metal hydroxide such assodium hydroxide or potassium hydroxide.

Thirdly, the carbonic acid ester [III] will be hereunder explained indetail.

The monohydric alcohols (A) preferably used herein are those having 1-15carbon atoms and are exemplified by those having a straight- orbranched-chain alkyl group, such as methanol ethanol, propanol, butanol,pentanol, hexanol, heptanol, octanol, nonanol, decanol, undecanol,dodecanol, tridecanol, tetradecanol or pentadecanol. More preferred aremonohydric alcohols having 1-9 carbon atoms.

The dihydric alcohols (B) preferably used herein are those having 2-12carbon atoms and are exemplified by ethylene glycol, 1,3-propanediol,propylene glycol, 1,4-butanediol, 1,2-butanediol,2-methyl-1,3-propanediol, 1,5-pentanediol, neopentyl glycol,1,6-hexanediol, 2-ethyl-2-methyl-1,3-propanediol, 1,7-heptanediol,2-methyl-2-propyl-1,3-propanediol, 2,2-diethyl-1,3-propanediol,1,8-octanediol, 1,9-nonanediol, 1,10-decanediol, 1,11-undecanediol and1,12-dodecanediol. More preferred are dihydric alcohols having 2-9carbon atoms.

The neopentyl type polyols (C) having 5-7 carbon atoms and 3-4 hydroxylgroups, or dimers or trimers thereof are exemplified bytrimethylolethane, trimethylolpropane, trimethylolbutane,pentaerythritol and a dimer or trimer thereof. Among these polyols (C),the most preferred are trimethylolpropane, di-(trimethylolpropane),tri-(trimethylolpropane), pentaerythritol, di-(pentaerythritol),tri-(pentaerythritol) and the like.

The carbonic acid esters [III] used as a base oil in a lubricating oilof the present invention are those each having a chemical structurewhich is originated from the above alcoholic components (A), (B) and(C). In other words, the carbonic acid esters [III] include not onlythose prepared by the synthesis of, as raw materials, carbonic acid andthe components (A), (B) and (C), but also those which resulted in havingthe chemical structure mentioned above even if the raw materials usedare not the same as the above.

The method for producing the carbonic acid ester [III] used in thepresent invention is not particularly limited. For example, the carbonicacid ester [III] can be produced by the reaction of a carbonic diesteror phosgene, as a source of carbonic acid moieties, with a mixture ofthe alcoholic components (A), (B) and (C), in the presence of an alkalisuch as metallic sodium, sodium hydroxide or sodium methoxide and at atemperature of 80°-200° C. In this case, various carbonic acid esters[III] having different chemical structures, molecular weights andviscosities can be obtained by changing the molar ratio of the carbonicacid moieties used to the alcoholic components used therewith. It isdesirable that these raw materials be reacted together in molecularratios of 1-100 moles, preferably 1-20 moles of a source of carbonicacid moieties, 1-20 moles, preferably 1-10 moles, of the component (A)and 1-100 moles, preferably 1-20 moles, of the component (B)respectively to 1 mole of the component (C).

In a case where the component (C) used is trimethylolpropane, thechemical structures of the resulting carbonic acid esters [III] areexemplified by the general formula (10) ##STR7## wherein R is an alkylgroup having 1-15 carbon atoms, R' is an alkylene group having 2-12carbon atoms and X is an integer of 0-12.

Further, the synthetic lubricating oils of the present invention will behereunder explained in more detail.

The carbonic acid esters [I]-[III] obtained by the methods as mentionedabove may be refined to remove the by-products and/or unreactedreactants, but the by-products and/or unreacted reactants may be presentin small amounts in the lubricating oils of the present invention as faras they do not impair the excellent performances thereof.

In the preparation of the synthetic lubricating oils of the presentinvention, the carbonic acid esters [I]-[III] mentioned above may beused singly, or jointly as a mixture of at least two kinds of theesters.

The molecular weight of each of the carbonic acid esters [I]-[III]according to the present invention is not particularly limited, but, ina case where the carbonic acid ester is used as a refrigerator oil, thenumber average molecular weight of the carbonic acid ester is in therange of preferably 200-3000, more preferably 300-2000, to improve thecompressor in sealability.

The kinematic viscosities of the carbonic acid esters [I]-[III]according to the present invention are in the range of preferably 2-150cSt, more preferably 4-100 cSt at 100° C.

The lubricating oil of the present invention may comprise as the onlybase oil at least one member selected from the above carbonic acidesters [I]-[III] and, as required, it may additionally comprise otherbase oils for lubricating oils. The other base oils will be illustratedhereinafter. For example, mineral oils usable as the other base oils areexemplified by paraffinic or naphthenic base oils which are produced bysubjecting lubricating oil fractions obtained by atmosphericdistillation or reduced pressure distillation of a crude oil, to asuitable combination of refining treatments such as solventdeasphalting, solvent extraction, hydrocracking, solvent dewaxing,catalytic dewaxing, hydrorefining, sulfuric acid treatment and claytreatment. Further, the synthetic oils usable as the other base oils areexemplified by poly alpha-olefins such as polybutene, 1-octene oligomersand 1-decene oligomers; alkylbenzenes; alkylnaphthalenes; diesters suchas ditridecyl glutarate, di 2-ethylhexyl adipate, diisodecyl adipate,ditridecyl adipate and di 3-ethylhexyl sebacate; polyol esters such astrimethylolpropane caprylate, trimethylolpropane pelargonate,pentaerythritol 2-ethyl hexanoate and pentaerythritol pelargonate;polyalkylene glycols; polyphenyl ethers; silicone oils; perfluoroalkylethers; and mixtures of at least two kinds of the synthetic oils.In a case where the other base oils are to be incorporated in thecarbonic acid esters [I]-[III], it is desirable that the carbonic acidesters [I]-[III] be present therein in a ratio of not less than 50% byweight, preferably not less than 70% by weight of the total amount ofthe other base oils and the carbonic acid esters.

In a case where the lubricating oil of the present invention is to beused as a refrigerator oil, the lubricating oil may comprise as the onlybase oil at least one member selected from the above carbonic acidesters [I]-[III] and, as required, it may additionally comprise otherbase oils for refrigerator oils. Among the other base oils, preferableones are illustrated as follows:

(a) a polyoxyalkylene glycol or an ether thereof represented by thegeneral formula (3) ##STR8## wherein R⁸ and R⁹ are each a hydrogen atomor an alkyl group having 1-18 carbon atoms, R¹⁰ is an alkylene grouphaving 2-4 carbon atoms, and f is an integer of 5-70,

(b) a polyoxyalkylene glycol glycerol ether represented by the generalformula (4) ##STR9## wherein R¹¹ -R¹³ are each a hydrogen atom or analkyl group having 1-18 carbon atoms, R¹⁴ -R¹⁶ are each an alkylenegroup having 2-4 carbon atoms, and g-i are each an integer of 5-7,

(c) an ester represented by the general formula (5) ##STR10## wherein X²is a group represented by the general formula --OR¹⁹ or ##STR11## X³ isa group represented by the general formula ##STR12## R¹⁷ and R²³ areeach an alkylene group having 1-8 carbon atoms, R¹⁸ and R²⁰ are each analkylene group having 2-16 carbon atoms, R¹⁹ and R²⁴ are each an alkylgroup having 1-15 carbon atoms, R²¹ and R²² are each an alkyl grouphaving 1-14 carbon atoms, j and k are each an integer of 0 or 1 and l isan integer of 0-30,

(d) a polyol ester represented by the general formula (6) ##STR13##wherein X⁴ is a group selected from the group consisting of methylgroup, ethyl group, propyl group and groups represented by the generalformula ##STR14## R²⁵ -R²⁸ may be identical with, or different from,each other and are each a group selected from the group consisting ofstraight-chain alkyl groups having 3-11 carbon atoms, branched-chainalkyl groups having 3-15 carbon atoms and cycloalkyl groups having 6-12carbon atoms, the straight-chain alkyl groups being present in a ratioof not more than 60% of the total alkyl groups, and m is an integer of1-3, and

(e) a polyol dicarboxylic acid ester represented by the general formula(7) ##STR15## wherein X⁵ and X⁶ may be identical with, or differentfrom, each other and are each a group selected from the group consistingof methyl group, ethyl group, propyl group and groups represented by thegeneral formula R²⁹ -R³³ may be identical with, or different from eachother and are each an alkyl group having 3-15 carbon atoms, R³⁴ is adivalent hydrocarbon group having 1-8 carbon atoms, and n is an integerof 1-5.

These other base oils represented by the formulae (3)-(7) may be usedsingly or jointly for adding to the lubricating oil of this invention.Further, the lubricating oil of this invention may be incorporated withparaffinic mineral oils, naphthenic mineral oils, polyα-olefins,alkylbenzenes and the like, but, in this case, the resulting mixed oilwill be lowered in miscibility with chlorine-free type halogenocarbons.

The amount of these other base oils incorporated is not particularlylimited as far as the excellent performances of the lubricating oil ofthis invention as a refrigerator oil are not impaired, but the otherbase oils should be present in the resulting mixed oil in a ratio ofusually less than 50% by weight, preferably not more than 30% by weightof the total amount of the mixed oil.

To further enhance the lubricating oil of the present invention inexcellent performances, the lubricating oil may be incorporated, asrequired, with heretofore known additives for lubricating oils. Theadditives are exemplified by antioxidants such as phenol-type ones,amine-type ones, sulfur-type ones, zinc thiophosphate-type ones andphenothiazine-type ones; wear resistant additives such as molybdenumdithiophosphate, molybdenum dithiocarbamate, molybdenum disulfide,carbon fluorides, boric esters, aliphatic amines, higher alcohols,higher fatty acids, fatty acid esters and fatty acid amides; extremepressure agents such as tricresyl phosphate, triphenyl phosphate andzinc dithiophosphate; corrosion inhibitors such as petroleum sulfonates,alkylbenzene sulfonates and dinonylnaphthalene sulfonates; metalinactivators such as benzotriazole; metallic detergents such as alkalineearth metal sulfonates, alkaline earth metal phenates, alkaline earthmetal salicylates and alkaline earth metal phosphonate; ash-freedispersants such as succinimide, succinate esters and benzylamine;antifoaming agents such as silicone; viscosity index improvers such aspolymethacrylate, polyisobutylene and polystyrene; and pour pointdepressants. These additives may be used singly or jointly. The contentsof the viscosity index improver, the antifoaming agent, the metalinactivator and each of the other additives are ordinarily 1-30% byweight, 0.0005-1% by weight, 0.005-1% by weight and 0.1-15% by weight ofthe total amount of the lubricating oil, respectively.

In a case where the lubricating oil of the present invention is to beused as a refrigerator oil, the lubricating oil may be incorporatedfurther with at least one kind of a phosphorous compound selected fromthe group consisting of phosphoric esters, acid phosphoric esters, aminesalts of acid phosphoric esters, chlorinated phosphoric esters andphosphorous esters, to improve the oil in wear resistance and loadresistance. These phosphorous compounds are esters of phosphoric acid orphosphorous acid and an alkanol or a polyether type alcohol, orderivatives of the esters. The phosphoric esters are exemplified bytributyl phosphate, triphenyl phosphate and tricresyl phosphate. Theacid phosphoric esters are exemplified by dietetradecyl acid phosphate,dipentadecyl acid phosphate, dihexadecyl acid phosphate, diheptadecylacid phosphate and dioctadecyl acid phosphate. The amine salts of acidphosphoric esters are exemplified by salts of the above acid phosphoricesters and amines such as methylamine, ethylamine, propylamine,butylamine, pentylamine, hexylamine, heptylamine, octylamine,dimethylamine, diethylamine, dipropylamine, dibutylamine, dipentylamine,dihexylamine, diheptylamine, dioctylamine, trimethylamine,triethylamine, tripropylamine, tributylamine, tripentylamine,trihexylamine, triheptylamine and trioctylamine. The chlorinatedphosphoric esters are exemplified by tris-dichloropropyl phosphate, trischloroethyl phosphate, polyoxyalkylene bis[di(chloroalkyl)] phosphateand tris chlorophenyl phosphate. The phosphorous esters are exemplifiedby dibutyl phosphite, tributyl phosphite, dipentyl phosphite, tripentylphosphite, dihexyl phosphite, trihexyl phosphite, diheptyl phosphite,triheptyl phosphite, dioctyl phosphite, trioctyl phosphite, dinonylphosphite, didecyl phosphite, diundecyl phosphite, triundecyl phosphite,didodecyl phosphite, tridodecyl phosphite, diphenyl phosphite, triphenylphosphite, dicresyl phosphite, tricresyl phosphite and mixtures thereof.These phosphorous compounds may be added to the lubricating oil in aratio of 0.1-5.0% by weight, preferably 0.2-2.0% by weight, of the totalamount of the lubricating oil.

In the case in which the lubricating oil of the present invention is tobe used as a refrigerator oil, to further improve the lubricating oil instability, it may be incorporated with at least one kind of an epoxycompound selected from the group consisting of phenylglycidyl ether typeepoxy compounds, glycidyl ester type epoxy compounds, epoxidized fattyacid monoesters and epoxidized vegetable oils. The said phenylglycidylether type epoxy compounds used herein include phenylglycidyl ether andalkylphenylglycidyl ethers. The said alkylphenylglycidyl ethers arethose having 1 to 3 alkyl groups having 1 to 13 carbon atoms, amongwhich are preferred those having an alkyl group having 4 to 10 carbonatoms, such as butylphenylglycidyl ether, pentylphenylglycidyl ether,hexylphenylglycidyl ether, heptylphenylglycidyl ether,octylphenylglycidyl ether, nonylphenylglycidyl ether anddecylphenylglycidyl ether. The said glycidyl ester type epoxy compoundsinclude phenylglycidyl esters, alkylglycidyl esters and alkenylglycidylesters, with glycidyl benzoate, glycidyl acrylate, glycidyl methacrylateand the like being preferred.

The epoxidized fatty acid monoesters include esters of an epoxidizedfatty acid having 12 to 20 carbon atoms and an alcohol having 1 to 8carbon atoms, phenol or an alkylphenol. In particular, butyl, hexyl,benzyl, cyclohexyl, methoxyethyl, octyl, phenyl or butylphenyl esters ofepoxidized stearic acid may preferably be used.

The epoxidized vegetable oils include epoxidized compounds of vegetableoils such as soybean oil, linseed oil and cottonseed oil.

Among these epoxy compounds, the preferable ones include phenylglycidylether type epoxy compounds and epoxidized fatty acid monoesters with theformer being more preferable. The most preferred are phenylglycidylether, butylphenylglycidyl ether and mixtures thereof.

In the case in which these epoxy compounds are to be incorporated in thelubricating oil of the present invention, it is desirable that they beincorporated therein in a ratio of 0.1-5.0% by weight, preferably0.2-2.0% by weight, of the total amount of the lubricating oil.

Furthermore, in a case where the lubricating oil of the presentinvention is to be used as a refrigerator oil, to further improve thelubricating oil in wear resistance and load resistance, it may beincorporated with a carboxylic acid represented by the general formula(8)

    R.sup.35 --CH(COOH).sub.2                                  ( 8)

and/or the general formula (9)

    R.sup.36 --CH.sub.2 --COOH                                 (9)

wherein R³⁵ and R³⁶ may be identical with, or different from, each otherand are each an alkyl group having 8-18 carbon atoms. The abovecarboxylic acids are exemplified by octylmalonic acid, nonylmalonicacid, decylmalonic acid, undecylmalonic acid, dodecylmalonic acid,tridecylmalonic acid, tetradecylmalonic acid, pentadecylmalonic acid,hexadecylmalonic acid, heptadecylmalonic acid, octadecylmalonic acid,decanoic acid, undecanoic acid, lauric acid, tridecanoic acid, myristicacid, pentadecanoic acid, palmitic acid, heptadecanoic acid, stearicacid, nonadecanoic acid, eicosanoic acid and mixtures of at least twokinds of the above carboxylic acids. In a case where these carboxylicacids are to be incorporated in the lubricating oil of the presentinvention, it is desirable that they be incorporated therein in a ratioof 0.01-3% by weight, preferably 0.05-2% by weight, of the total amountof the lubricating oil.

Of course, at least two kinds of the aforementioned phosphoruscompounds, epoxy compound and carboxylic acids may be used jointly.

In the case where the lubricating oil of the present invention is to beused as a refrigerator oil, to further enhance the lubricating oil inperformances, the lubricating oil may be incorporated, as required, withheretofore known additives for a refrigerator oil, which includephenol-type antioxidants such as di-tert-butyl-p-cresol and bisphenol A;amine-type antioxidants such as phenyl-α-naphthylamine andN,N-di(2-naphthyl)-p-phenylenediamine; wear resistant additives such aszinc dithiophosphate; extreme pressure agents such as chlorinatedparaffin and sulfur compounds; oiliness improvers such as fatty acids;antifoaming agents such as silicone-type ones; and metal inactivatorssuch as benzotriazole. These additives may be used singly or jointly.The total amount of these additives added is ordinarily not more than10% by weight, preferably not more than 5% by weight, of the totalamount of the lubricating oil.

In the case in which conventional refrigerator oils and/or additives areto be incorporated in the carbonic acid esters [I]-[III] of the presentinvention, it is desirable that the carbonic acid esters [I]-[III] beincorporated in the resulting lubricating oil in a ratio of more than50% by weight, preferably not less than 70% by weight, of the totalamount of the lubricating oil.

In a case where the lubricating oil of the present invention comprisingat least one kind of the carbonic acid esters [I]-[III] as a majorcomponent is to be used as a refrigerator oil, the lubricating oilshould have such viscosity and pour point as those which are normallysuitable for an ordinary refrigerator oil, but it should desirably havea pour point of not higher than -10° C., preferably -20° C. to -80° C.,to prevent it from solidification at a low temperature. Further, itshould desirably have a kinematic viscosity of not less than 2 cSt,preferably not less than 3 cSt at 100° C., to keep the sealability ofthe compressor of the refrigerator when used, while it should desirablyhave a kinematic viscosity of not more than 150 cSt, preferably not morethan 100 cSt at 100° C., in view of its fluidity at a low temperatureand the efficiency of heat exchange in the evaporator when used.

The refrigerants which may be used in refrigerators in which thelubricating oils of the present invention are suitably used asrefrigerator oils, include hydrogen-containing halogenocarbons such as1,1,2,2-tetrafluoroethane (HFC-134), 1,1,1,2-tetrafluoroethane(HFC-134a), 1,1-difluoroethane (HFC-152a), trifluoromethane (HFC-23),monochlorodifluoromethane (HCFC-22) or 1-chloro-1,1-difluoroethane(HCFC-142b); hydrogen-free halogenocarbons such astrichloromonofluoromethane(CFC-11), dichlorodifluoromethane (CFC-12),monochlorotrifluoromethane (CFC-13) and monochloropentafluoromethane(CFC-115); and mixtures of at least two of the halogenocarbons. Amongthese halogenocarbons, the hydrogen-containing halogenocarbons arepreferable with HFC-134a being particularly preferable in view of theenvironmental problems.

The lubricating oils of the present invention are very excellent inmiscibility with the hydrogen-containing halogenocarbons, particularlychlorine-free type halogenocarbons, as compared with the heretoforeknown lubricating oils. Further, the lubricating oils of the presentinvention are excellent ones which have not only high miscibility withthe chlorine-free type halogenocarbons and high electrical insulatingproperty but also high lubricity and low hygroscopicity.

The lubricating oils of the present invention may particularlypreferably be used as refrigerator oils in air-conditioners,dehumidifiers, cold-storage chests, freezers, freeze and refrigerationwarehouses, automatic vending machines, showcases, cooling units inchemical plants, and the like which have a reciprocating or rotarycompressor. Further, the above lubricating oils may also preferably beused in refrigerators having a centrifugal compressor. Further, thelubricating oils of the present invention may preferably be used as notonly the refrigerator oils, but also engine oils, gear oils, hydraulicoils, metal processing oils and the other industrial lubricating oils.

The present invention will be better understood by the followingExamples and Comparative Examples, but the present invention is notlimited to the embodiments described in the Examples.

EXAMPLES 1-9 AND COMPARATIVE EXAMPLES 1-5

Lubricating oils of the present invention and comparative lubricatingoils of conventional types are illustrated as follows:

[Example 1] A synthetic lubricating oil comprising a carbonic acid esterhaving a branched structure represented by the formula (11): ##STR16##

[Example 2] A synthetic lubricating oil comprising a carbonic acid esterhaving a branched structure represented by the formula (12): ##STR17##

[Example 3] A synthetic lubricating oil comprising a carbonic acid esterhaving a branched structure represented by the formula (13): ##STR18##

[Example 4] A synthetic lubricating oil comprising a carbonic acid esterhaving a branched structure represented by the formula (14): ##STR19##

[Example 5] A synthetic lubricating oil comprising a carbonic acid esterhaving a branched structure represented by the formula (15): ##STR20##

[Example 6] Ten (10) moles of diethyl carbonate, 4 moles of2-methyl-1-pentanol, 3 moles of neopentyl glycol and 1 mole oftrimethylolpropane were introduced into a reactor and then subjected totransesterification in the presence of metallic sodium as a catalyst at120° C. for 4 hours to obtain a reaction mixture. The thus obtainedreaction mixture was distilled to remove ethanol formed by thetransesterification therefrom, treated with diluted hydrochloric acid,washed with water and then distilled again to remove low molecularweight components in the reaction product therefrom to obtain a test oilwhich was a mixture of carbonic acid esters according to the presentinvention.

[Example 7] Ten (10) moles of diethyl carbonate, 4 moles of2,2,4-trimethyl-1-pentanol, 3 moles of 3-methylpentanediol and 1 mole oftrimethylolpropane were introduced into a reactor and then subjected totransesterification in the presence of metallic sodium as a catalyst at120° C. for 4 hours to obtain a reaction mixture. The thus obtainedreaction mixture was distilled to remove ethanol formed by thetransesterification therefrom, treated with diluted hydrochloric acid,washed with water and then distilled again to remove low molecularweight components in the reaction product therefrom to obtain a test oilwhich was a mixture of carbonic acid esters according to the presentinvention.

[Example 8] Ten (10) moles of diethyl carbonate, 2 moles of2-methyl-1-pentanol, 2 moles of 3-methylpentanediol, 2 moles ofneopentyl glycol and 1 mole of pentaerythritol were introduced into areactor and then subjected to transesterification in the presence ofmetallic sodium as a catalyst at 120° C. for 4 hours to obtain areaction mixture. The thus obtained reaction mixture was distilled toremove ethanol formed by the transesterification therefrom, treated withdiluted hydrochloric acid, washed with water and then distilled again toremove low molecular weight components in the reaction product therefromto obtain a test oil which was a mixture of carbonic acid estersaccording to the present invention.

[Examples 9] Ten (10) moles of diethyl carbonate, 4 moles of2,2,4-trimethyl-1-pentanol, 4 moles of propylene glycol and 1 mole ofpentaerythritol were introduced into a reactor and then subjected totransesterification in the presence of metallic sodium as a catalyst at120° C. for 4 hours to obtain a reaction mixture. The thus obtainedreaction mixture was distilled to remove ethanol formed by thetransesterification therefrom, treated with diluted hydrochloric acid,washed with water and then distilled again to remove low molecularweight components in the reaction product therefrom to obtain a test oilwhich was a mixture of carbonic acid esters

[Comparative Example 1] A naphthenic mineral oil

[Comparative Example 2] A branched-chain type alkylbenzene (averagemolecular weight: about 300)

[Comparative Example 3] A tetraester of pentaerythritol,2-methyl-hexanoic acid and 2,4-dimethylheptanoic acid

[Comparative Example 4] A complex ester of 3-methyl-1,5-pentanediol,adipic acid and 3,5,5-trimethylhexanoic acid (average molecular weight:about 500)

[Comparative Example 5] Polyoxypropylene glycol (average molecularweight: about 900)

The base oils of Examples 1-9 for the lubricating oils of the presentinvention were evaluated for their performances that were theirmiscibility with HFC-134a, hydrolysis resistance, insulating property,wear resistance and hygroscopicity by the following respective testmethods. For comparison, the mineral oil, the alkylbenzene, the esteroils and the polyoxyalkylene glycol of the Comparative Examples 1-5which have heretofore been used as lubricating oils were evaluated inthe same manner as in Examples 1-9.

Miscibility with HFC-134a

0.2 g of the test oil of each of the Examples and the ComparativeExamples and 2.0 g of the refrigerant (HFC-134a) were sealed in a glasstube having an inner diameter of 6 mm and a length of 220 mm. The glasstubes so sealed were placed in a thermostat maintained at apredetermined low temperature and then in a thermostat kept at apredetermined high temperature to observe whether the refrigerant andthe test oil were miscible with each other, separated from each other ormade white-turbid.

Hydrolysis test

Sixty grams (60 g) of each of the test oils and 0.6 g of water wereintroduced in a 200-ml glass test tube and copper, iron and aluminumplates (6 cm²) were then placed therein as deterioration acceleratingcatalysts, after which the whole in the tube was heated to 175° C. for168 hours in an autoclave made of stainless steel thereby to thermallydeteriorate the test oils.

After the test, each of the test oils was measured for acid value andhydroxyl value.

Insulating property

The test oils were each measured for specific volume resistivity at 25°C. in accordance with JIS C 2101.

FALEX wear test

The test oils were each applied to a test journal for measuring theamount of the test journal worn by having the journal run in at a testoil temperature of 100° C. under a load of 150 lb for 1 minute and thenrunning it under a load of 250 lb for 2 hours in accordance with ASTM D2670.

Hygroscopicity

Thirty grams (30 g) of each of the test oils were placed in a 300-mlbeaker, allowed to stand for 7 days in an air-conditioned bathmaintained at a temperature of 30° C. and a humidity of 60% and thenmeasured for moisture content by the Karl-Fischer method.

                                      TABLE 1                                     __________________________________________________________________________           Kinematic                                                                           Miscibility with HFC-134a                                                                  Hydrolysis resistance                                                                          Falex test                         Example-                                                                             viscosity                                                                           Miscible     (mg KOH/g) Resistivity                                                                         Amount of                                                                            Hygroscopicity              Comparative                                                                          @ 100° C.                                                                    temperature range                                                                          Acid Hydroxyl                                                                            @ 25° C.                                                                     journal worn                                                                         30° C. 60%           Example                                                                              (cSt) (°C.) value                                                                              value (Ω · cm)                                                             (mg)   (%)                         __________________________________________________________________________    Example 1                                                                            4.9   <-35-90      0.05 2.6   3.1 × 10.sup.14                                                               18     0.18                        Example 2                                                                            5.2   <-30-82      0.04 2.2   2.5 × 10.sup.14                                                               15     0.12                        Example 3                                                                            5.1   <-35-76      0.05 2.8   2.8 × 10.sup.14                                                               20     0.18                        Example 4                                                                            4.7   <-30-68      0.05 2.5   2.9 × 10.sup.14                                                               17     0.12                        Example 5                                                                            11.2  <-20-51      0.07 1.0   8.5 × 10.sup.13                                                               12     0.09                        Example 6                                                                            5.5   <-70-80      0.02 2.5   3.0 × 10.sup.14                                                               21     0.19                        Example 7                                                                            8.2   <-70-80      0.03 1.9   2.5 × 10.sup.14                                                               18     0.15                        Example 8                                                                            8.9   <-70- 80     0.05 1.1   2.2 × 10.sup.14                                                               18     0.10                        Example 9                                                                            12.5  <-80-80      0.03 0.7   1.8 × 10.sup.14                                                               11     0.09                        Comparative                                                                          5.1   Immiscible   0.11 0.0   3.8 × 10.sup.15                                                               23     0.01                        Example 1                                                                     Comparative                                                                          4.8   Immiscible   0.05 0.0   6.4 × 10.sup.15                                                               25     0.01                        Example 2                                                                     Comparative                                                                          5.2   <-45-95      9.5  11.3  1.1 × 10.sup.14                                                               14     0.19                        Example 3                                                                     Comparative                                                                          6.8   <-50-82      15.2 18.2  1.2 × 10.sup.14                                                               20     0.22                        Example 4                                                                     Comparative                                                                          9.6   <-60-64      0.15 --    2.6 × 10.sup.10                                                               38     2.90                        Example 5                                                                     __________________________________________________________________________

It is apparent from the results indicated in Table 1 that thelubricating oils (Examples 1-9) of the present invention are veryexcellent in miscibility with a refrigerant, HFC-134a, as compared withthose of Comparative Example 1-2.

Table 1 also shows that the ester oils of Comparative Examples 3 and 4are excellent in miscibility with the refrigerant and insulatingproperty, but they are inferior in hydrolysis resistance whereby theywill raise a problem as to corrosion by acids formed when used in arefrigerator system in which external moisture is expected to enter intothe system and mix with the ester oils, while the ester oils of Examples1-9 will not raise any problem because of no formation of acids althoughthey are somewhat hydrolyzable and form hydroxyl groups when used insuch a refrigerator system as above.

As for insulating property, the lubricating oils of Examples 1-9 are byno means inferior to mineral oils and alkylbenzenes, are almost equal toester oils and are excellent as compared with the alkylene glycol oil ofComparative Example 5.

The FALEX wear test shows that the lubricating oils of Examples 1-9 areat least equal in wear resistance to those of Comparative Examples 1-5.

As for hygroscopicity, the lubricating oils of Examples 1-9 are by nomeans inferior to the mineral oil of Comparative Example 1 and thealkylbenzene of Comparative Example 2, are almost equal to the esteroils of Comparative Examples 3-4 and are very excellent as compared withthe alkylene glycol of Comparative Example 5.

Effect of the Invention

As is apparent from the above comparative experiments, the lubricatingoils of the present invention are excellent in electrical insulatingproperty as well as wear resistance, nonhygroscopicity and hydrolysisresistance, and are suitable for use in refrigerators using therein ahydrogen-containing halogenocarbon as a refrigerant. They can thereforebe preferably used particularly as a refrigerator oil.

What is claimed is:
 1. A synthetic lubricating oil comprising as a baseoil a mixture of (i) at least one carbonic acid ester selected from thegroup consisting of:[I] a carbonic acid ester represented by the generalformula (1) ##STR21## wherein R¹ and R² are identical with, or differentfrom each other and are each a group selected from the group consistingof alkyl groups having 1-15 carbon atoms and dihydric alcohol residueshaving 2-12 carbon atoms, R³ is an alkylene group having 2-12 carbonatoms, and a is an integer of 1-30; [II] a carbonic acid esterrepresented by the general formula (2) ##STR22## wherein X¹ is ahydrogen atom or a group represented by the general formula ##STR23## R⁵and R⁶ are identical with, or different from, one another and are each agroup selected from the group consisting of alkyl groups having 1-12carbon atoms, cycloalkyl groups having 5-12 carbon atoms and neopentyltype polyol residues having 5-30 carbon atoms and 3-8 hydroxyl groups;R⁷ is an alkyl group having 1-6 carbon atoms; b is an integer of 1 or 2,c is an integer of 0 or 1 with the proviso that the sum of b and cequals to 2; d is an integer of 0-30, at least one of R⁴ and R⁵indicating a neopentyl type polyol residue when d is 0; and e is aninteger of 1-3; and [III] a carbonic acid ester other than said carbonicacid esters [I] and [II], which is prepared from a source of carbonicacid moieties and the following alcoholic components: (A) a monohydricalcohol, (B) a dihydric alcohol, and (C) a neopentyl type polyol having5-7 carbon atoms and 3-4 hydroxyl groups, or a dimer or trimer of thepolyol, and(ii) at least one oil selected from the group consisting of(a) a polyoxyalkylene glycol or an ether thereof represented by thegeneral formula (3) ##STR24## wherein R⁸ and R⁹ are each a hydrogen atomor an alkyl group having 1-18 carbon atoms, R¹⁰ is an alkylene grouphaving 2-4 carbon atoms, and f is an integer of 5-70. (b) apolyoxyalkylene glycol glycerol ether represented by the general formula(4) ##STR25## wherein R¹¹ -R¹³ are each a hydrogen atom or an alkylgroup having 1-18 carbon atoms, R¹⁴ -R¹⁶ are each an alkylene grouphaving 2-4 carbon atoms, and g-i are each an integer of 5-7. (c) anester represented by the general formula (5) ##STR26## wherein X² is agroup represented by the general formula --OR¹⁹ or ##STR27## X³ is agroup represented by the general formula ##STR28## R¹⁷ and R²³ are eachan alkylene group having 1-8 carbon atoms, R¹⁸ and R²⁰ are each analkylene group having 2-16 carbon atoms, R¹⁹ and R²⁴ are each an alkylgroup having 1-15 carbon atoms, R²¹ and R²² are each an alkyl grouphaving 1-14 carbon atoms, j and k are each an integer of 0 or 1 and ρ isan integer of 0-30. (d) a polyol ester represented by the generalformula ##STR29## wherein X⁴ is a group selected from the groupconsisting of methyl group, ethyl group, propyl group and groupsrepresented by the general formula ##STR30## R²⁵ -R²⁸ may be identicalwith, or different from, each other and are each a group selected fromthe group consisting of straight-chain alkyl groups having 3-11 carbonatoms, branched-chain alkyl groups having 3-15 carbon atoms andcycloalkyl groups having 6-12 carbon atoms, the straight-chain alkylgroups being present in a ratio of not more than 60% of the total alkylgroups, and m is an integer of 1-3, and (e) a polyol dicarboxylic acidester represented by the general formula (7) ##STR31## wherein X⁵ and X⁶may be identical with, or different from, each other and are each agroup selected from the group consisting of methyl group, ethyl group,propyl group and groups represented by the general formula ##STR32## R²⁹-R³³ may be identical with, or different from each other and are each analkyl group having 3-15 carbon atoms, R³⁴ is a divalent hydrocarbongroup having 1-8 carbon atoms, and n is an integer of 1-5.
 2. Asynthetic lubricating oil according to claim 1, wherein the alcoholiccomponent (C) of the carbonic acid ester [III] is at least one memberselected from the group consisting of trimethylolpropane,di-(trimethylolpropane), tri-(trimethylolpropane), pentaerythritol,di-(pentaerythritol) and tri-(pentaerythritol).
 3. A syntheticlubricating oil according to claim 1, wherein the synthetic lubricatingoil is used as a lubricating oil for refrigerators.
 4. A syntheticlubricating oil according to claim 2, wherein the synthetic lubricatingoil is used as a lubricating oil for refrigerators.
 5. A syntheticlubricating oil according to claim 3, wherein the halogenocarbon used asa refrigerant in the refrigerators is a hydrogen-containing one.
 6. Asynthetic lubricating oil according to claim 4, wherein thehalogenocarbon used as a refrigerant in the refrigerators is ahydrogen-containing one.
 7. A synthetic lubricating oil according toclaim 1, wherein the base oil is a mixture of:(e) a polyol dicarboxylicacid ester represented by the general formula (7) ##STR33## wherein X⁵and X⁶ may be identical with, or different from, each other and are eacha group selected from the group consisting of methyl group, ethyl group,propyl group and groups represented by the general formula ##STR34## R²⁹-R³³ may be identical with, or different from each other and are each analkyl group having 3-15 carbon atoms, R³⁴ is a divalent hydrocarbongroup having 1-8 carbon atoms, and n is an integer of 1-5.
 8. Asynthetic lubricating oil according to claim 1, wherein the at least onecarbonic acid ester (i) is comprised in an amount of more than 50% byweight based on the total amount of the synthetic lubricating oil.
 9. Asynthetic lubricating oil according to claim 7, wherein the at least onecarbonic acid ester (i) is comprised in an amount of more than 50% byweight based on the total amount of the synthetic lubricating oil.
 10. Asynthetic lubricating oil according to claim 1, further comprising atleast one kind of a phosphorus compound selected from the groupconsisting of phosphoric esters, acid phosphoric esters, amine salts ofacid phosphoric esters, chlorinated phosphoric esters and phosphorousesters, in an amount of 0.1-5.0% by weight based on the total amount ofthe synthetic lubricating oil.
 11. A synthetic lubricating oil accordingto claim 7, further comprising at least one phosphorus compound selectedfrom the group consisting of phosphoric esters, acid phosphoric esters,amine salts of acid phosphoric esters, chlorinated phosphoric esters andphosphorous esters, in an amount of 0.1-5.0% by weight based on thetotal amount of the synthetic lubricating oil.
 12. A syntheticlubricating oil according to claim 1, further comprising at least oneepoxy compound selected from the group consisting of phenylglycidylether type epoxy compounds, glycidyl ester type epoxy compounds,epoxidized fatty acid monoesters and epoxidized vegetable oils, in anamount of 0.1-5.0% by weight based on the total amount of the syntheticlubricating oil.
 13. A synthetic lubricating oil according to claim 7,further comprising at least one epoxy compound selected from the groupconsisting of phenylglycidyl ether type epoxy compounds, glycidyl estertype epoxy compounds, epoxidized fatty acid monoesters and epoxidizedvegetable oils, in an amount of 0.1-5.0% by weight based on the totalamount of the synthetic lubricating oil.
 14. A synthetic lubricating oilaccording to claim 1, further comprising a carboxylic acid representedby the general formula (8)

    R.sup.35 --CH(COOH).sub.2                                  ( 8)

and/or the general formula (9)

    R.sup.36 --CH.sub.2 --COOH                                 (9)

wherein R³⁵ and R³⁶ may be identical with, or different from, each otherand are each an alkyl group having 8-18 carbon atoms, in an amount of0.01-3% by weight based on the total amount of the synthetic lubricatingoil.
 15. A synthetic lubricating oil according to claim 7, furthercomprising a carboxylic acid represented by the general formula (8)

    R.sup.35 --CH(COOH).sub.2                                  ( 8)

and/or the general formula (9)

    R.sup.36 --CH.sub.2 --COOH                                 (9)

wherein R³⁵ and R³⁶ may be identical with, or different from, each otherand are each an alkyl group having 8-18 carbon atoms, in an amount of0.01-3% by weight based on the total amount of the synthetic lubricatingoil.
 16. The lubricating oil according to claim 1 wherein said carbonicacid ester has the formula ##STR35## wherein R is isoheptyl.
 17. Thelubricating oil according to claim 1 wherein said carbonic acid esterhas the formula ##STR36## wherein R is isohexyl or isoheptyl.
 18. Thelubricating oil according to claim 1 wherein said carbonic acid esterhas the formula ##STR37## wherein R is isopropyl or isobutyl.
 19. Thelubricating oil according to claim 1 wherein said base oil has a numberaverage molecular weight of 200-3000, a kinematic viscosity of 2-150 cStat 100° C.